The present invention generally relates to active implantable medical devices (AIMDs). More particularly, the present invention relates to an EMI shielded conduit for leads extending from the hermetic feedthrough terminal of an active implantable medical device to a remote electronic circuit board, substrate or network located within the AIMD hermetically sealed and electromagnetically shielded housing.
Feedthrough hermetic terminals are generally well-known in the art for connecting electrical signals through the housing or case of an AIMD such as those illustrated in FIG. 1. For example, in implantable medical devices such as cardiac pacemakers, shown in FIG. 2, implantable cardioverter defibrillators, and the like, a hermetic terminal 100 comprises one or more conductive terminal pins 102a-102d supported by an insulative structure for feedthrough passage from the exterior to the interior of an AIMD electromagnetic shield housing 104. Many different insulator structures and related mounting methods are known in the art for use in AIMDs, wherein the insulative structure also provides a hermetic seal to prevent entry of body fluids into the housing of the AIMD. However, feedthrough terminal pins are typically connected to one or more implanted leads 106 and 106′ are routed from the outside or body fluid side of the AIMD electromagnetic shield housing 104 to cardiac tissues such as those located in a right atrium 108 or in a right ventricle 110. These implanted leads 106 undesirably act as an antenna and thus tend to collect stray electromagnetic interference (EMI) signals from the patient environment for conducted transmission into the interior of the AIMD electromagnetic shield housing 104. Such EMI signals may interfere with the proper operation of AIMD electronic circuits. In general, the AIMD internal electronic circuits are located on a circuit board or substrate 112. Also shown is an RF telemetry pin 114. This acts as a telecommunications antenna coupled from the outside of the AIMD electromagnetic shield housing 104 and the hermetic terminal 100 to the interior of the AIMD to a telecommunications circuit board 116. It will be obvious to those skilled in the art that the circuit board 112 and the telecommunications circuit board 116 can be combined into one overall substrate or they may be broken down into several different substrates located within the AIMD electromagnetic shield housing 104. In many prior art devices, as shown in FIG. 2, there is a ceramic feedthrough filter capacitor 118 which is typical of many prior art devices. In this case the hermetic terminal 100 has been combined directly with frequency selective components such as the ceramic feedthrough filter capacitor 118 or MLCC chip capacitors or the like (not shown), to decouple or divert interfering signals from the point of lead ingress to the shielded housing of the AIMD. Examples of mounting MLCC chip capacitors to the hermetic terminals of AIMDs are more thoroughly described in U.S. Pat. Nos. 5,650,759 and 5,896,267, the contents of which are incorporated herein by reference. It is very important to decouple these signals at their point of ingress to the electromagnetically shielded AIMD so that such stray signals do not re-radiate or couple to sensitive circuits inside the AIMD electromagnetic shield housing 102. FIG. 3 is an example of good practice in the mounting of a prior art broadband EMI filter 120 such as the ceramic feedthrough capacitor 118. In this case, the broadband EMI filter 120 has been mounted directly to or adjacent to the hermetic terminal 100 at the point of ingress of an implanted lead 122. A leadwire 124 routed inside the AIMD electromagnetic shield housing 104 are free of high frequency EMI signals. Accordingly, the leadwires 124 cannot reradiate or couple undesirably to internal circuit board 112 electronic components. FIG. 4 is an example of prior art poor practice. In this case, the onboard EMI filter components have been mounted on the internal circuit board 112 inside of the AIMD electromagnetic shield housing 104. By locating the filtering on the internal circuit board 112, this presents a low impedance which tends to pull an undesirable EMI signal 126 that couples to the implanted lead 122 inside the AIMD electromagnetic shield housing 104. These EMI signals 126 can then reradiate as EMI re-radiation from the internal leadwires 124. It has been shown in the past that such re-radiation can cause AIMD internal electronic circuit malfunction.
Using a cardiac pacemaker as an example, the AIMD electromagnetic shield housing 104 is typically made of titanium, stainless steel, or other suitable biocompatible material which creates an equipotential shield housing. Seams are uniformly laser welded so that there are no openings. An alternative is use of a ceramic, plastic or composite housing with an electromagnetic shield coating disposed on either its interior and/or exterior surfaces. The AIMD electromagnetic shield housing 104 may also be coated with nano materials that form an RF shield. The AIMD electromagnetic shield housing 104 provides hermeticity to protect the sensitive electronic circuits from the intrusion of body fluids.
At high frequencies, the AIMD electromagnetic shield housing 104 both reflects and absorbs incident electromagnetic waves. For example, the evolution and design of such electromagnetically shielded titanium housings have made pacemakers relatively immune to microwave ovens and other high frequency interference sources. The AIMD electromagnetic shield housing 104 also forms a very convenient equipotential surface to which high frequency EMI signals conducted from the implanted leads 122 may be decoupled/diverted. This is typically done using passive or active filter elements which can be mounted directly on or adjacent to the point of AIMD housing implanted lead 122 ingress. In the prior art, the optimal location is to place such bypass (lowpass) filters on or adjacent to the hermetic feedthrough pin terminal. The ceramic feedthrough filter capacitors 118 are typically mounted on the hermetic terminal 100 and provide a low impedance at high frequencies from the leadwires 124 to the AIMD electromagnetic shield housing 104, thereby shorting or diverting high frequency EMI signals to the housing 104. When the high frequency EMI energy is diverted to the AIMD electromagnetic shield housing 104, it simply circulates as eddy currents resulting in a few milliwatts of insignificant power dissipation as a small amount of heat. This results in a miniscule and insignificant temperature rise of the AIMD electromagnetic shield housing 104.
Other (early) prior art designs attempted to provide effective filtering by providing on-board or circuit board substrate mounted low pass EMI filter elements. For example, Intermedics Corporation attempted to use MLCC chip capacitors mounted on a flex cable and/or circuit board or substrate near where the pacemaker sensing amplifiers and microcircuits were placed. Although, the filters did their job and acted as a low impedance, they tended to pull stray EMI RF currents from the outside world to the point of filtering. Because these filters were connected at the end of the flex cable or a leadwire inside of the AIMD housing, these stray EMI signals tended to radiate from the flex cable/leadwires and cross-couple to other sensitive electronics inside the AIMD housing. FIG. 5 is an illustration of the attempt by Intermedics to place an MLCC chip capacitor 128 on a flex cable 130 located within the AIMD electromagnetic shield housing 104. The flex cable 130 was unshielded but could have a ground circuit trace 132. It also has an active circuit trace 134. The MLCC chip capacitor 128 was located between a pair of electrical connections 136a, 136b between the ground circuit trace 132 and the active circuit trace 134. Unfortunately, this created an inductive loop 136 which very effectively reradiated EMI inside the AIMD electromagnetic shield housing 104. In the prior art, this is a dramatic illustration of the need to place filter components directly next to the hermetic terminal 100 connected between a leadwire 124 and a ferrule 138 so that no loops such as the inductive loop 136 are formed but reradiate EMI. This is why the prior art feedthrough capacitor as illustrated in FIGS. 2-13 are generally shown mounted directly to the ferrule 138 and the hermetic terminal 100 of the AIMD. It is a basic principle of good EMI filter engineering that filters be placed at a point of entry to a shielded housing where they can immediately decouple the stray EMI signals to the housing or overall shield of the electronics module.
Therefore it has become common to locate the EMI filters directly at the hermetic terminal 100 which is the point of ingress of the implanted lead 122 from the outside world (body fluid side) to the inside of the AIMD electromagnetic shield housing 104.
As used herein, the term lead, which is synonymous with implanted lead, shall mean the lead or leads that are routed from the exterior of the AIMD electromagnetic shield housing 104 into body tissues. The term leadwire refers to wiring, flex cables or circuit traces inside of the AIMD electromagnetic shield housing 104.
Moreover, as used herein, the term remote, as applied to a circuit board, substrate, capacitor, low pass filter, L-C trap filter, electronic filter, bandstop filter, high voltage suppression array, diode array, and/or short to housing switch network shall mean that any combination of these circuits are mounted remotely relative to the AIMD hermetic seal at or near the distal end of the shielded conduit (usually mounted on an AIMD circuit board or substrate). In general, the novel shielded conduit will both shield the leadwires routed from the AIMD hermetic terminal to the circuit board, remote substrate, capacitor, low pass filter, L-C trap filter, electronic filter, bandstop filter, high voltage suppression array, diode array, and/or short to housing switch network and at the same time also provide a low impedance RF ground return to the overall AIMD electromagnetic shield housing 104 for said remote circuits.
EMI filtered feedthrough hermetic terminals are shown and described in U.S. Pat. No. 4,424,551, U.S. Pat. No. 5,333,095, U.S. Pat. No. 5,905,627, U.S. Pat. No. 5,973,906, U.S. Pat. No. 5,959,829, and U.S. Pat. No. 5,759,197, the contents of which are incorporated herein. There are a number of problems associated with low pass EMI feedthrough filters mounted directly to the hermetic terminal 100, including increased cost, masking a hermetic seal leaker, and reduced reliability.
Cost is increased because it is difficult to reliably mount the filter elements (such as a multipin feedthrough capacitor) on or immediately adjacent to the hermetic terminal subassembly. The hermetic terminal subassembly itself is generally constructed at very high temperature. For example, with a gold brazed alumina hermetic seal subassembly, the gold brazing to the ferrule and terminal pins is done at approximately 800° C. If it is a glass seal composite ceramic subassembly, again very high temperatures are required to re-flow the glass. The subsequent mounting of the EMI filter element is typically done at much lower temperatures. However, when the filter is mounted directly to or against the hermetic terminal subassembly, it is subjected to significant installation stresses. The pacemaker manufacturer generally laser welds the ferrule of the hermetic seal subassembly into the titanium housing of the AIMD. This creates both a thermal shock, thermal rise and mechanical stress which the prior art filter elements must be able to withstand. Installation of these filter elements to the hermetic seal typically involves expensive silver-filled thermal-setting conductive flexible adhesives such as polyimides. In addition to being expensive, these materials are difficult to dispose into the correct positions and require carefully fixturing and cleaning operations. An additional cost comes from the fact that these life-saving devices go through high reliability screening. This includes thermal shock and burn-in of the electronic elements including the low pass filter hermetic seal subassembly. Failure of the electronic filter means that the entire hermetic seal subassembly is also scrap. This is an expensive proposition due to the fact that hermetic terminal subassembly is typically manufactured with biocompatible materials including platinum, platinum iridium pins, gold brazes and the like. In other words, a significant amount of the cost of this subassembly is due to the precious metals involved in its design.
Therefore, a significant problem in the prior art relates to the mounting of feedthrough or MLCC capacitors and other types of EMI filters on or adjacent to a hermetic terminal pin/seal subassembly. In the prior art, such hermetic seals are very carefully tested after installation into the AIMD housing to ensure that they meet a maximum hermetic seal leak rate. This is generally a 1×10−7 or 1×10−8 cc per second maximum leak rate. The mounting of a low pass EMI filter assembly generally involves thermal-setting conductive adhesives, epoxies and bonding washers which can mask a leaking hermetic seal. There are a number of patents that allow channels or paths for leak detection including U.S. Pat. No. 6,566,978, the contents of which are incorporated herein. However, these channels/paths are difficult to manufacture and also add cost.
FIG. 1 illustrates various types of active implantable and external medical devices 140a-140i that are currently in use. FIG. 1 is a wire formed diagram of a generic human body showing a number of implanted medical devices. Numeral 140a represents a family of hearing devices which can include the group of cochlear implants, piezoelectric sound bridge transducers and the like. Numeral 140b represents a variety of neurostimulators and brain stimulators. Neurostimulators are used to stimulate the Vagus nerve, for example, to treat epilepsy, obesity and depression. Brain stimulators are pacemaker-like devices and include electrodes implanted deep into the brain for sensing the onset of the seizure and also providing electrical stimulation to brain tissue to prevent a seizure from actually occurring. The leadwires associated with a deep brain stimulator are often placed using real time MRI imaging. Most commonly such leadwires are placed during real time MRI. Numeral 140c shows a cardiac pacemaker which is well-known in the art. Numeral 140d includes the family of left ventricular assist devices (LVAD's), and artificial hearts, including the recently introduced artificial heart known as the Abiocor. Numeral 140e includes an entire family of drug pumps which can be used for dispensing of insulin, chemotherapy drugs, pain medications and the like. Insulin pumps are evolving from passive devices to ones that have sensors and closed loop systems. That is, real time monitoring of blood sugar levels will occur. These devices tend to be more sensitive to EMI than passive pumps that have no sense circuitry or externally implanted leadwires. Numeral 140f includes a variety of bone growth stimulators for rapid healing of fractures. Numeral 140g includes urinary incontinence devices. Numeral 140h includes the family of pain relief spinal cord stimulators and anti-tremor stimulators. Numeral 140h also includes an entire family of other types of neurostimulators used to block pain. Numeral 140i includes a family of implantable cardioverter defibrillator (ICD) devices and also includes the family of congestive heart failure devices (CHF). This is also known in the art as cardio resynchronization therapy devices, otherwise known as CRT devices.
FIG. 6 is a prior art unipolar discoidal feedthrough capacitor 142, which has an active internal electrode plate set 144 and a ground electrode plate set 146. An inside diameter termination surface 148 is connected electrically to the active electrode plate set 144. An outside diameter termination surface 150 is both solderable and electrically conductive, and it is connected to the ground electrode plate set 146.
FIG. 7 is a cross-section of the unipolar discoidal feedthrough capacitor 142 of FIG. 6 shown mounted to a feedthrough hermetic terminal 152 of an active implantable medical device (AIMD). In prior art discoidal feedthrough capacitor devices 142, the leadwire 124 is continuous. The feedthrough hermetic terminal 152 is attached to, typically, a titanium housing 154, for example, of a cardiac pacemaker. An insulator 156, like alumina ceramic or glass, is disposed within the ferrule 138 and forms a hermetic seal against body fluids. The leadwire 124 extends through the feedthrough hermetic terminal 152, passing through aligned passageways through the insulator 156 and the unipolar discoidal feedthrough capacitor 142. A gold braze 158 forms a hermetic seal joint between the leadwire 124 and the insulator 156. A second gold braze 160 forms a hermetic seal joint between the alumina insulator 156 and the ferrule 138. A laser weld 162 provides a hermetic seal joint between the ferrule 138 and the titanium housing 154. The unipolar discoidal feedthrough capacitor 142 is shown surface mounted in accordance with U.S. Pat. No. 5,333,095, and has an electrical connection 164 between its inside diameter metallization 148 and hence the active electrode plate set 144 and the leadwire 124. There is also an outside diameter electrical connection 164 which connects to the outside diameter metallization 150 and hence the ground electrode plate set 146 to the ferrule 138. Feedthrough capacitors are very efficient high frequency devices that have minimal series inductance. This allows them to operate as EMI filters over very broad frequency ranges.
Referring once again to FIG. 7, one can see that another way to describe a prior art unipolar discoidal feedthrough capacitor 142 is as a three-terminal capacitor. Three-terminal devices generally act as transmission lines. One can see that there is a current 168 that passes into the leadwire 124. On the body fluid side there is generally an implanted lead 122 which can undesirably act as an antenna which can pick up energy from environmental emitters. This energy is known as electromagnetic interference (EMI). Cell phones, microwave ovens and the like have all been implicated in causing interference with active implantable medical devices. If this interference enters the leadwire 124 at a first terminal 170 (FIG. 7), it is attenuated along its length by the unipolar discoidal feedthrough capacitor 142. Upon exiting, the undesirable high frequency EMI has been cleaned off of the normal low frequency circuit current (such as pacemaker pacing pulses or biologic frequency sensors) so that the high frequency EMI has been significantly attenuated. Another way of looking at this is as the high frequency energy passes from the first terminal 170 to a second terminal 172 (FIGS. 7 and 8), it is diverted through the unipolar discoidal feedthrough capacitor 142 to a ground terminal 174 which is also known as the third terminal or terminal 3. In this case, the ground terminal 174 is the connection to the overall electromagnetically shielded housing of the AIMD. The unipolar discoidal feedthrough capacitor 142 of FIGS. 6-8, diverts unwanted high frequency EMI signals from the leadwire 124 to the AIMD electromagnetic shield housing 104 of the AIMD where it dissipates as a few milliwatts of harmless thermal energy.
The unipolar discoidal feedthrough capacitor 142 also performs two other important functions: (a) the internal active electrodes 144 and the internal ground electrodes 146 act as a continuous part of the overall electromagnetic shield housing of the electronic device or module which physically blocks direct entry of high frequency RF energy through the feedthrough hermetic terminal 152 or equivalent opening for leadwire ingress and egress in the otherwise completely shielded housing (such RF energy, if it does penetrate inside the shielded housing can couple to and interfere with sensitive electronic circuitry), and; (b) the unipolar discoidal feedthrough capacitor 142 very effectively shunts undesired high frequency EMI signals off of the leadwires 124 to the overall shield housing where such energy is dissipated in eddy currents resulting in a very small temperature rise.
FIG. 8 is a schematic diagram showing the unipolar discoidal feedthrough capacitor 142 previously described in connection with FIGS. 6 and 7. As one can see, it is a three-terminal device consistent with the first terminal 170, the second terminal 172 and the ground terminal 174 illustrated in FIG. 7.
FIG. 9 is a quadpolar prior art feedthrough capacitor 176 which is similar in construction to that previously described in FIG. 6 except that it has four through holes.
FIG. 10 is a cross-section showing the internal active electrodes 144 and ground electrodes 146 of the quadpolar feedthrough capacitor 176 of FIG. 9.
FIG. 11 is a schematic diagram showing the four discrete feedthrough capacitors comprising the quadpolar feedthrough capacitor 176 of FIGS. 9 and 10.
FIG. 12 is an exploded electrode view showing the inner and outer diameter electrodes of the unipolar discoidal feedthrough capacitor 142 of FIGS. 6 and 7. One can see the active electrode plates set 144 and the ground electrode plate set 146. A cover layer 178 is put on the top and bottom for added electrical installation and mechanical strength.
FIG. 13 is an exploded view of the interior electrodes of the prior art quadpolar feedthrough capacitor 176 of FIG. 9. The active electrode plate sets are shown as 144 and the ground electrode plates are shown as 146. The cover layers 178 serve the same purpose as previously described in connection with FIG. 12.
FIG. 14 is a prior art monolithic ceramic capacitor (MLCC) 180. These are made by the hundreds of millions per day to service consumer electronics and other markets. Virtually all computers, cell phones and other types of electronic devices have many of these. One can see that the MLCC 180 has a body 182 generally consisting of a high dielectric constant ceramic such as barium titinate. It also has a pair of solderable termination surfaces 184, 184′ at either end. These solderable termination surfaces 184, 184′ provide a convenient way to make a connection to the internal electrode plates 144, 146 of the MLCC capacitor 180. FIG. 14 can also take the shape and characteristics of a number of other types of capacitor technologies, including rectangular, cylindrical, round, tantalum, aluminum electrolytic, stacked film or any other type of capacitor technology.
FIG. 15 is a sectional view taken from section 15-15 in FIG. 14. The MCLL 180 includes a left hand electrode plate set 186 and a right hand electrode plate set 188. One can see that the left hand electrode plate set 186 is electrically connected to the external metallization surface 184. The opposite, right hand electrode plate set 188 is shown connected to the external metallization surface 184′. Prior art MLCC 180 and equivalent chip capacitors are also known as two-terminal capacitors. That is, there are only two ways electrical energy can connect to the body of the capacitor. In FIGS. 14 and 15, the first terminal 170 is on the left side and the second terminal 172 is on the right side.
FIG. 16 is the schematic diagram of the MLCC chip capacitor 180 illustrated in FIGS. 14 and 15.
FIG. 17 is a different type of prior art MLCC feedthrough capacitor 180 that is built into a special configuration known in the art by some as a flat-through capacitor 190. At low frequencies, the flat-through capacitor 190 exhibits ideal capacitance behavior versus frequency. That is, its attenuation curve versus frequency is nearly ideal. This is because it is truly a three-terminal device which acts as a transmission line in a manner similar to those of prior art unipolar discoidal feedthrough capacitors 142. This is better understood by referring to its internal electrode plate geometry as shown in FIG. 18, wherein a through or active electrode plate 144 is sandwiched between two ground electrode plates 146, 146′. The through or active electrode plate 144 is connected at both ends by termination surfaces 184 and 184′. When the flat-through capacitor 190 is mounted between the circuit trace lands 192 and 192′ as shown in FIG. 17, this connects the circuit trace together between points 192 and 192′. Referring to the active electrode plate 144 in FIG. 18, one can see the current 168 entering. If this is a high frequency EMI current, it will be attenuated along its length by the capacitance of the flat-through capacitor 190 and emerge as a much smaller in amplitude EMI signal at the second terminal 172 as 168′. Similar to the unipolar discoidal feedthrough capacitor 142, the flat-through capacitor 190 is also a three-terminal capacitor as illustrated in FIG. 17. The point where the current 168 ingresses is at the first terminal 170. The point where the circuit current 170′ egresses is known as the second terminal 172. Lastly, the ground terminal 174 is known as the third terminal. In other words, any RF currents that are flowing down the circuit trace lands 192 must pass through the active electrode plate 144 of the flathrough capacitor 190. This means that any RF signals are exposed for the full length of the active electrode plate 144 between the ground electrode plates 146, 146′ and the capacitance that is formed between them. This has the effect of making a very novel shape for a three-terminal feedthrough capacitor. One negative to this type of flat-through capacitor 190 is that it is not conveniently mountable in such a way that it becomes an integral part of an overall shield.
There is always a frequency at which an undesirable RF coupling 194 (FIG. 17) across the device will occur. This usually does not happen until 100 MHz or above. At very high frequencies, such as above 1 GHz, this problem becomes quite serious. Another negative, as compared to the prior art unipolar discoidal feedthrough capacitor 142 (where the circuit current 168 passes through a robust leadwire 124 in a feedthrough hole), is that the flat-through capacitor 190 circuit current 168 must flow through the active electrodes 144 of the flat-through capacitor 190 itself (in the prior art unipolar discoidal feedthrough capacitor 142, the only current 168 that flows in the active electrodes 144 is high frequency EMI currents). Monolithic ceramic manufacturing limitations on electrode thickness and conductivity means that the prior art flat-through capacitors 190 have relatively high series resistance and can only be rated to a few hundred milliamps or a few amps at best (however, an implantable defibrillator must deliver a high voltage pulse of over 20-amps). Prior art MLCC and flat-through electrodes must be kept relatively thin to promote ceramic grain growth through the electrodes in order to keep the capacitor layers from delaminating during manufacturing or worse yet, during subsequent mechanical or thermal shocks which can cause latent failures.
FIG. 19 is a schematic diagram of the prior art flat-through capacitor 190 shown in FIG. 17. Note that this schematic diagram is the same as that for the unipolar discoidal feedthrough capacitor 142 shown in FIGS. 6 and 7. The difference is that the unipolar discoidal feedthrough capacitor 142 is inherently configured to be mounted as an integral part of an overall shield which precludes the problem of the RF coupling 194 (see FIGS. 9-11).
FIG. 20 illustrates the attenuation versus frequency response curve which is shown generically for the flat-through capacitor 192 of FIG. 17. If it were not for cross-coupling of RF energy, it would perform as an ideal or nearly perfect capacitor. However, because of this cross-coupling, there is always going to be a certain frequency at which the attenuation starts to parasitically drop off as shown. This drop off is very undesirable in active implantable medical device (AIMD) applications in that there would be less protection against high frequency EMI emitters such as cellular phones and the like. This parasitic drop off in attenuation due to cross-coupling is even a worse problem in military and space applications where EMI filter attenuation requirements of up to 10 or even 18 GHz, is important (implantable medical applications do not generally require filtering much above 3 GHz due to the effective reflection and absorption of human skin of RF energy at frequencies above 3 GHz). Space and military circuits have to operate in the presence of extremely high frequency emitters, such as GHz radars and the like.
Accordingly, there is a need for an EMI shielded conduit assembly for an active implantable medical device, and a related method of remotely mounting the low pass EMI filter or other electronic component or assembly at a location remote from the hermetic terminal subassembly. Ideally, this mounting would be either at or near the circuit board or substrate where automated low cost electronic assembly methods could be used. Moreover, there is a need for providing a shielded conduit which has the effect of extending the overall electromagnetic shield (titanium housing) to a remote location at the location of the low pass filter. Specifically, in the case of a feedthrough capacitor, there is a need for it to be mounted inside of the conduit or even on the circuit board. Ideally, the shield should extend all the way to the low pass filter elements such that no coupling or re-radiation inside of the device can occur. The present invention fulfills these needs and provides other benefits.